Using a slag conditioner to beneficiate bag house dust from a steel making furnace

ABSTRACT

A slag conditioner containing MgO, up to 50% slag-making carbon, 10 to 60% particulates of bag house dust and dropout box particles and 2% to 25% binder is mixed and formed under pressure to produce aggregates which can have the form of a briquette. The MgO content of the mixture comprising: 20% to 90% burned aggregates comprised of particles less than 8 mm of which at least 30% is 0.2 mm or greater and containing between 35% and 94% MgO; and up to 50% light burned magnesite.

CROSS-REFERENCE TO RELATED APPLICATIONS

U.S. patent application Ser. No. 10\990,678 filed Nov. 17, 2004

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to using particulate emissions essentiallyincluding oxides of iron, calcium, silicon, magnesium, zinc, cadmium andlead collected by an emission containment system for a steel makingfurnace as an essential component in a slag conditioner in the steelmaking furnace and, more particularly, to using such particulateemissions from an earlier steelmaking operation in an electric arcfurnace as one of the major components of a slag conditioner for theelectric arc furnace in on going steel making operations to increase theyield of the steel by the furnace and enrich the particulate emissionswith greater quantities of oxides of zinc, lead and cadmium in bag housedust to allow disposal with enhanced commercial value.

2. Description of the Prior Art

The particulate emissions occurring during the operation of electric arcfurnaces are contained by a system of ducts that yield particulatebyproducts collected in one or more dropout boxes and a bag house. Whenthe furnace charge is comprised of scrap, the particulate emissions areidentified as a hazardous material classified by the EPA as KO61 dust.Typically, KO61 dust is generated at a rate of between 20 and 45 lbs ofdust per ton produced by the electric furnace. The variance to the dustgeneration rates depends on the particle size of the lime and the methodused to introduce lime products into the furnace during steel makingprocess. When the lime is added to the furnace charge, typically in theform of iron bearing scrap, the lime is of sufficient size in excess of½ inch aggregates to avoid air borne loss by a negative pressure appliedby the exhaust of the bag house collection system. As a typical example,the dust generation rate falls to between 20 to 28 lbs. per ton ofsteel. When the lime is injected by pneumatic transportation, the limeis very small finer than ½ inch with some fines and has much more dustis easily pulled into the evacuation system and ends in the bag housewhich accounts for the large difference in bag house dust generation, ashigh as 45 lbs per ton of steel produced. The chemistry of bag housedust classified by the EPA as KO61 is site dependent and the followingtwo typical examples demonstrate the existence of potential usefulcommodities. One example of bag house dust generated at a rate of 28-30pounds per ton of steel from steel scrap and charged lime containedprincipal chemical components of: 16% CaO; 35% Fe₂O₃; 6% SiO₂; 03% MgO;2% Al₂O₃; 3% carbon; 28% ZnO; and 7% other. A second example of baghouse dust generated at a rate of 38-40 pounds per ton of steel from asteel scrap charge and injected lime contained principal chemicalcomponents of: 25% CaO; 29% Fe₂O₃; 5% SiO₂; 2% MgO; 2% Al₂O₃; 6% carbon;25% ZnO; and 6% other.

It is known in the art to introduce bag house dust with as part of thescrap furnace charge into a steel making electric furnace. However, theparticulate size distribution as typically illustrated by the graph ofFIG. 1 is unfavorable to the steel making process since generally the90% of the particles are less than 100 microns and about 50% are lessthan 10 microns. As a result it is believed a major portion of the dustparticulate is drawn off by the emission containment system withoutparticipating as a beneficial part of the furnace charge. The operationof the electric steel making furnace alters the chemistry of theresulting bag house dust and the slag. The chemistry of the scrap makingup the furnace charges will usually vary with each particular furnacecharge. However, the typically chemistry for both the KO61 dust and slagmade during the operation of an electric steel making furnace using thedust as part of the furnace charge are for the KO61 dust: CaO 22.1%;Fe₂O₃ 36.3%; SiO₂ 6.1%; MgO 3.3%; Al₂O₃ 2.0%; Carbon 1.3%; ZnO 24.5%;and Other 2.0%. The chemistry for the slag is: FeO 41.0%; SiO₂ 11.7%;CaO 26.2%; Al₂O₃ 5.32%; MgO 7.9%; MnO 5.88%; Cr₂O₃ 2.28%; TiO₂ 0.37%;P₂O₅ 0.10%; and SO₃ 0.16%. Comparing these to chemicals reveals that theabsence of zinc content of the slag is believed to occur because thezinc vaporizes and recovered as part of the KO61 dust. The presentinvention is based on the realization that the KO61 dust and slagcontain an abundance of oxides that can be altered by the use of slagconditioner of the present invention to benefit the entire operation ofthe electric steel making furnace. For example, it can be seen that theiron oxide content of the dust and slag is remarkably high and whensuccessfully chemically reduced will increase the yield of steel by theelectric furnace. The MgO content of the dust can be beneficiallyreduced in the dust to increase the MgO content in the slag whereby theresulting slag composition is affected for chemically preventing erosionof the furnace refractory and the usual foaming of the slag. The calciumoxide content of the dust can be reduced to exercise control of thelime-silica ratio.

Those skilled in the art of steel making were aware of the implicationof the slag composition related to refractory life. It is known fromphase diagrams that magnesium oxide is soluble in calcium silicate basedliquid slag and that the solubility level depended primarily on the CaOto SiO2 ratio (“C/S”), commonly referred to as lime-silica ratio. Whenthe lime-silica ratio in the slag composition at the end of a heat wasgreater than 2/1, the slag was found to have a chemical imbalancerequiring about 7% MgO to be satisfied. A relationship exists betweenlining material wear and MgO content of the slag. The maintenancematerials of the furnace lining have a high MgO content and becamesacrificial donors of the deficient amounts of dissolving MgO to theslag, the damage to the lining limited the vessel campaign to between400 to 1200 heats. Steelmakers, who added burned limestone for the CaOcomponent in the basic slag, began to add burned dolomite or a blend ofthe burned limestone and burned dolomite to supply not only CaO but alsoMgO as a slag addition to satisfy the demand for MgO in the slag.Refractory lining life was improved but wear remained a continuingproblem requiring frequent refractory relining which interrupted steelproduction.

In the 1980's the lining life of Basic Oxygen steel Furnaces (BOF)vessels was improved by changing the composition of slag for the steelmaking operations by increasing MgO content which made the slag moreviscous. The presents of viscous slag combined with the gas blowingcapabilities of the BOF, resulted in a practice called slag splashing. Acoating of the viscous refractory slag blown onto the furnace wallsprotected the vessel lining from excessive wear and was practiced afteralmost every heat. The steelmaking process could be carried out usingthe renewed slag coating on the refractory lining after each heat. Thisslag coating process extended the lining life in some instances to morethan 10,000 heats in BOF furnaces and attempts were made to apply asimilar concept of a slag coating process to the electric arc furnace.U.S. Pat. No. 6,514,312, issued Feb. 4, 2003 contains a disclosure ofthe slag splashing practice to a BOF vessel.

Accordingly, it is an object of the present invention to provide auseful slag conditioner composition having an MgO component of at least15% and a size particulate thereof above 0.2 mm for a steel makingfurnace using particulate emissions of such furnace particularly baghouse dust as recycled major component from a furnace facility,particularly the same on site steel making furnace facility.

It is another object of the present invention to provide a useful slagconditioner composition having an MgO component comprised of dolomitealone or without added carbon bearing material at least 15% particulatethereof above 0.2 mm and a water free binder for a steel making furnaceusing particulate emissions including bag house dust as recycled majorcomponent from a furnace facility, particularly the same on site steelmaking furnace facility.

It is a further object of the present invention to provide an improvedbriquette first mention of agglomeration, briquetting preferredcomposition useful to extract oxides, particularly Ca, Si, Fe, Fe₂, andMg for dispersion in a slag volume as an enhancement to the chemistrythereof and saturating the slag with an MgO content to impart a creamyand easily foamed properties to the slag and a carbon content tochemically reduce iron oxides in the furnace burden and the developingslag including such oxides in the introduced slag conditioner toincrease the productivity of the steel making furnace.

It is a further object of the present invention to provide an improvedbriquette composition or other formed agglomerate including KO61 dustand/or drop out box material and capable of accommodating changes inKO61 chemistry and changes with the KO61 product or blend of materialswith one of a plurality of different quantities of such recycled dustand/or material selected according to a varying chemistry andconcentrations, disposal value, availability to neutralize the oxides ofthe slag and reduce the iron as a slag conditioner for slag in anelectric steel making furnace.

It is a further object of the present invention to provide an improvedbriquette composition including KO61 dust and/or drop out box materialuseful to increase zinc, cadmium, and lead contaminants in particulatescollected as particulates classified as bag house dust for enhancing thecommercial value by a periodic routing a part of the dust in the recyclecircuit for sale or disposed of at a reduced cost to the steel makingoperation.

A further object of the present invention provides that the recycling ofparticulate emission including drop out box particulates and/or baghouse dust occurs in such quantities so as to match or reduce the dustgeneration rates of the steel making furnace and reduce the amount ofKO61 requiring disposal.

A further object of the present invention to chemically reduce an oxideof iron which are chemically bound in the KO61 material by combiningcarbon with the KO61 material in slag bath of a steel making furnace andultimately increase the yield of steel produced by the furnace and lowerthe dust generation rates, typically by 5 to 10 lbs per ton of steelproduced

A further object of the present invention is to provide an improved slagconditioner composition using refuse particulates of MgO and carbonbased slag conditioner selected from used crushed brick from either asteel making furnace or ladle therefore, lime, carbon; KO61 bag housematerial and or drop out box material; dead burned MgO; and bindingagents

Another object of the present invention is to alter a slag compositionin a steel making furnace by the addition of a select amount ofmagnesium oxide, a part of which is supplied by recycled quantities ofKO61 particulate material to more economically create useful slagproperties including an increased viscosity, creamy texture, and anincreased ease for foaming of the slag to provide a protective coatingon the furnace walls to extend the useful lining life.

SUMMARY OF THE INVENTION

More particularly according to the present invention there is provided aslag conditioner comprising by weight a mixture and 2% to 25% binder forbonded agglomerates or larger particles of the mixture, the mixturecomprising: 10% to 60% particulates comprised of bag house dustessentially containing oxides of iron, calcium, silicon, magnesium,zinc, lead and cadmium; 20% to 90% burned aggregates comprised ofparticles less than 8 mm of which at least 30% is 0.2 mm or greater andcontaining between 35% and 94% MgO; up to 50% slag-making carbonaceousadditive; and up to 50% light burned magnesite.

According to a further aspect of the present invention there is provideda method of making steel including the steps of recovering particulateemissions containing oxides of iron, calcium, silicon, magnesium, zinc,lead and cadmium from a particulate containment system including baghouse of a electric steel making furnace, forming briquettes comprisedof a mixture of aggregates and 15% to 60% of the particulate emissionsobtained by the step of recovering and 2% to 30% binder to agglomeratethe mixture, the mixture comprising: by weight 15% to 80% dead burnedmagnesite comprised of particles less than 8 mm of which at least 30% is0.2 mm or greater and containing between 35% and 94% MgO; up to 40%light burned magnesite; and 5% to 50% carbon selected from the groupconsisting of: coal; coke; graphite and petroleum coke and iron oxide,introducing an iron bearing charge into the electric steel makingfurnace, melting and refining an iron bearing charge in the electricfurnace while forming an overlying layer of slag, introducing the slagconditioner to the electric steelmaking furnace in an amount needed toraise the MgO level in the overlying layer of slag to between 5% to 14%and thereby impart a creamy slag texture, non leaching for soluble MgO,foam producing to increasing slag volume, and protectively coatrefractory sidewalls of the electric steelmaking furnace.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention will be more fully understood when the followingdescription is read in light of the accompanying drawings in which:

FIG. 1 is a graph to schematic illustrates a particle size spectrum in arandom sampling of KO61 particulate material;

FIG. 2 is a schematic illustration of a steel making operationincorporating the present invention; and

FIG. 3 is a flow diagram of the process for making slag conditioningagglomerates according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The chemical symbol MgO as used herein refers to products recovered fromroasting natural magnesite ore in an oven, one product called lightburned magnesite takes its name from the common process roasting the oreat a temperature of about 2000° F. for a desired period of time, andsecond product called dead burned magnesite takes its name from thecommon process roasting the ore at a temperature of about 3000° F. for adesired period of time. The compound MgO after dead burning develops anobservable crystal, periclase, which is chemically resistant to highlime (CaO) containing furnace slag. Also useful in compositions with MgObased refractory brick was chrome ore, which was added for thermal shockresistance for linings in electric arc and open hearth furnaces. Thepractice of using chemically basic linings caused a chemical change tothe slag floating on the steel. The chemical change was a higher lime(CaO) content to the slag for chemical compatible with the refractorylining materials. In addition, the high CaO content of the basic slagimproved a necessary metallurgical function of collecting undesirableimpurities from the liquid steel bath. Among these impurities betterremoved by basic slag are sulfur, phosphorus, and silicon depending onthe grade of steel being produced.

As used herein dead burned magnesite commonly referred to as DB MgO isan economical source of MgO in a crystalline form made up of aggregatesof periclase crystals, predominantly large crystals. These crystals arechemically known as magnesium oxide, MgO. Light burned magnesitecommonly referred to as LB MgO is also source of MgO, however themagnesium oxide crystals derived from light burned magnesite or lightburned dolomite are smaller and dissolve with greater ease in liquidslag than the magnesium oxide crystals of dead burned magnesite. The MgOconstituent in the slag must be sufficient to saturate the slag with MgOthereby prevent absorption of MgO from other sources in the furnace.However, quantities of MgO in excess of the stoichiometric amount are tobe present as a solid crystalline suspension to serve as a thickeningagent to increase the bulk viscosity of the slag and impart the desiredcreamy texture to the slag. The MgO held in suspension is mosteffectively supplied by the relatively larger crystals of MgO derivedfrom dead burned magnesite or dead burned dolomite.

As shown in FIG. 2, an electric steel making furnace 10 receives acharge of scrap material from a furnace charger 12 and during theoperations of the furnace for melting and refining the furnace chargethere also supplied a slag conditioner from a charger 14. Preferably thecomposition of the slag conditioner comprising by weight a mixture and2% to 25% binder for bonded agglomerates or larger particles of saidmixture, said mixture comprising: 20% to 90% burned aggregates comprisedof particles less than 8 mm of which at least 30% is 0.2 mm or greaterand containing between 35% and 94% MgO; up to 50% slag-makingcarbonaceous additive; and up to 50% light burned magnesite.

During the operation of the furnace, particulate emissions are collectedin an exhaust system maintained under negative atmospheric pressure forcollection of the particulates in a facility commonly referred to in theart as a bag house 16. At the conclusion of each such the steel makingoperation, slag is drained from the furnace to a slag pot 18 and refinedsteel burden is separately discharged from the furnace into a transportvessel 20. According to the present invention, particulates collected inthe bag house 16 are routinely removed to a first storage bin 22. Suchparticulates essentially include FeO, ZnO₂, CaO, SiO₂, MgO, Cd and Pb asemissions during the melting and refining of the scrap charge togetherwith the usual charges of carbonaceous and lime bearing materials. Anaccumulation of the particulates in the first storage bin occurringunder these circumstances will define a generally consistent chemicalcontent but will vary insignificantly with the site specific charge ofscrap to the furnace. After the particulate volume in the storage bin 22is used as a component in an ongoing formulation of slag conditionerbriquettes. The briquettes can be introduced as part of the initialfurnace charge and/or crushed and introduced as a first recyclingformulation of crushed slag conditioning briquettes by charger 14 to anoverlying layer of slag in a sufficient amount of to raise the MgO levelin slag to between 5% to 14% during melting and refining of an ironbearing charge in an electric steel making furnace.

The operation of the steel producing furnace using the first recyclingformulation of crushed slag conditioning briquettes also generatesparticulates collected in the bag house 16 are routinely removed to asecond storage bin 24 or added as replacement quantities to the storagebin 22. The second storage bin 24 is optional. The collection of suchparticulates will essentially contain ZnO₂, Cd and Pb as emissionsduring the ongoing operations of the steel making furnace Anaccumulation of the particulates in the second storage bin 24 occurringunder these circumstances will define a generally consistent chemicalcontent but will vary insignificantly with the site specific charge ofscrap to the furnace. The storage bin 22, and when used, storage bin 24will periodically receive quantities of dropout box particulates thatare collected from boxes placed at convenient locations about the ductfor the fume containment system of the electric steel making furnace.The particulates supplied by the drop out box notably include scale likeformations from the duct form generally of higher concentrations ofrelatively low melting point compounds. Essentially, however, the oxidessupplied by dropout particulates are the same as the oxides recoveredfrom the bag house. The briquettes are crushed and introduced as asecond recycling formulation of crushed slag conditioning briquettes bycharger 14 are altered by variations to the recycled particles andchange from a start up quantity used in the slag re-conditioner ofbetween 10 and 60% which is reduced as the recycle program continues tobetween 10 and 30% and to a minimum of 10 to 25%. The variance beingdependent on lower quantities of iron, calcium, silicon and magnesiumoxides and increased quantities of zinc, lead and cadmium to anoverlying layer of slag in a sufficient amount of to raise the MgO levelin slag to between 5% to 14% during melting and refining of an ironbearing charge in an electric steel making furnace.

The slag conditioner composition according to the present invention mayincorporate different size fractions of aggregate materials andformulations to supply MgO in an effectively sized crystalline form. Anunderlying discovery of the present invention is intermediate and finesized crystals of magnesium oxide can be added to a furnace in anagglomerate form and enter efficiently into chemical reactions withliquid slag phase without detrimental loss of smaller sizes of magnesiumoxide crystals to exhaust gases during the steel making process.Supplying MgO in the slag by the slag conditioner will save erosion ofMgO from the high cost refractory brick linings, gunning repair mixes,and prepared granular bottom repair mixes of the furnace linings. Anadequate MgO content in the slag also facilitates the use of relativelysmall amounts of carbon bearing materials to produce foaming of the slagfor protecting the refractory furnace lining and enhancing the operationof the steel making furnace. As used herein dead burned magnesitecommonly referred to as DB MgO is an economical source of MgO in acrystalline form made up of aggregates of periclase crystals,predominantly large crystals. These crystals are chemically known asmagnesium oxide, MgO. Light burned magnesite commonly referred to as LBMgO is also source of MgO, however the magnesium oxide crystals derivedfrom light burned magnesite are smaller and dissolve with greater easein liquid slag than the magnesium oxide crystals of dead burnedmagnesite. The MgO constituent in the slag must be sufficient tosaturate the slag with MgO thereby prevent absorption of MgO from othersources in the furnace. However, quantities of MgO in excess of thestoichiometric amount are to be present as a solid crystallinesuspension to serve as a thickening agent to increase the viscosity ofthe slag and impart the desired creamy texture to the slag. The MgO heldin suspension is most effectively supplied by the relatively largercrystals of MgO derived from dead burned magnesite or dead burneddolomite.

As shown in FIG. 3, there is a preferred arrangement of apparatus toformulate the slag conditioner of the present invention. As describedpreviously, there is loaded a 1^(st) MgO Source into a hopper 100 byweight 20% to 90% burned aggregates comprised of particles less than 8mm of which at least 30% is 0.2 mm or greater and containing between 35%and 94% MgO; up to 50% slag-making additive; and as a 2^(nd) MgO Sourceup to 50% light burned magnesite. The slag conditioner also essentiallyincludes between 10 and 60% which can be reduced to 10 to 30% but atleast 10 to 25% particulates including bag house dust a common wastebyproduct of a steel making furnace operation as a recycled majorcomponent from a furnace facility, particularly the same on site steelmaking furnace facility to effectively supply beneficial constituents tothe layer of slag forming during the ongoing steel making process andincrease the yield of steel by the furnace. Other components forming thegrope of suitable products for inclusion in the slag conditioner of thepresent invention are refuse particulates of MgO and carbon based slagconditioner selected from used crushed brick from either a steel makingfurnace or ladle therefore, lime, carbon; KO61 bag house material and ordrop out box material, fines of a bonded mixture containing MgO andcarbon such as maybe found in other slag conditioning agglomerates andfines of the agglomerates forming a slag conditioner of the presentinvention mixture.

The slag conditioner is formed as a briquette or other similaragglomerate or as an agglomerate crushed form such as briquettescomprised of an agglomeration of different size fractions of aggregatematerials including bag house dust and formulated to supply intermediateand fine sized crystals of magnesium oxide and enter efficiently intochemical reactions with liquid slag phase without detrimental loss ofsmaller sizes of magnesium oxide crystals and iron and calcium bearingcompounds to exhaust gases during the steel making process. During therefining process of the scrap charge in the electric steel makingfurnace the conditions in the slag and the steel burden allow a chemicalreaction to reduce oxide of iron which are chemically bound in the KO61material by combining carbon with the KO61 material in slag bath of asteel making furnace and ultimately increase the yield of steel producedby the furnace and lower the dust generation rates, typically by 5 to 10lbs per ton of steel produced. Supplying MgO in the slag by the slagconditioner will save erosion of MgO from the high cost refractory bricklinings, gunning repair mixes, and prepared granular bottom repair mixesof the furnace linings. An adequate MgO content in the slag alsofacilitates the use of relatively small amounts of carbon bearingmaterials to produce foaming of the slag for protecting the refractoryfurnace lining and enhancing the operation of the steel making furnace.The slag conditioner of the present invention will alter a slagcomposition in a steel making furnace by the addition of a select amountof magnesium oxide, a part of which is supplied by recycled quantitiesof KO61 particulate material to more economically create useful slagproperties including an increased viscosity, creamy texture, and anincreased ease for foaming of the slag to provide a protective coatingon the furnace walls to extend the useful lining life.

The slag making additive also introduced in the hopper 100, by weight,is carbonaceous; preferably having carbon content between 78% and 99.8%.The carbonaceous material preferably has a particle size of less than 6mm is useful and can be a size fraction of 5×0 mm, or a smaller sizefraction of 3×0 mm, but the smallest size fraction is 1×0 mm. Thecarbonaceous additive can be selected from the group comprising coal;anthracite coal; metallurgical coke; petroleum coke; graphite andpetroleum coke. A hopper 102 is supplied, by weight, with 2% to 25%binder for bonding agglomerates or larger particles of the mixture inhopper 100. The weighed quantity of binder can be a liquid such as wateror selected from the group consisting of: sodium silicate;ligosulfonate; lignosulfonate solutions; hydrochloric acid; sulfuricacid; magnesium chloride; magnesium sulphate; molasses; pitch; tar;asphalt; bentonite; clays and resins, with or without water added, eachwith sufficient liquid to form a moldable mixture. Alternate binders toreduce or essentially eliminate the hydroxide binder formed by as thereaction product of water with caustic MgO component of the conditionerwill not play a significant role in slag or steel making except to actas a temporary binder for the agglomerated particles, in one case, ofdead burned magnesium oxide and coal. Organic binders using 6% water orless are useful to make briquettes in compositions of this invention.Low ignition loss binders permit a higher weight percent of usefulsteelmaking materials, i.e. MgO and carbon units. Another advantage ofthe use of low ignition loss binders is that the energy required todecompose hydroxides and/or carbonates from slag conditioners in themelting process is minimized, if not eliminated. The slag conditioner inbriquette form can be designed to have a sufficiently low ignition lossso to be exothermic and thus will not deplete energy from the steelmaking furnace. Another advantage in the use of organic binders is thatthe need for light burned magnesite as a source for MgO can be replacedwith additional dead burned magnesium oxide fines, which are moreresistant to hydration, thereby making the life of slag conditionerbriquettes longer in storage. The use of organic binders providesanother advantage. Binders can be selected that contain little or nowater. In this case, alternate materials sensitive to hydration can beemployed in slag conditioners in the same particle size ranges. Thosealternate materials include but are not limited to burned dolomite anddead burned dolomite. In compositions based on burned dolomite in placeof dead burned magnesium oxide, the intermediate particles contributereactive sources of MgO and CaO, both oxides being useful for steelmaking slag to produce similar useful results as those compositionsbased on dead burned magnesium oxide. Some slag conditioner formulationsof the present invention provide that the burned aggregates are presentbetween 40% to 80% and, in such a formulation, the light burnedmagnesite is up to 40% and the binder is between 2% to 25%.

The size range of particles comprising the dead burned aggregate isfurther defined by a size fraction of 6×0 mm at least 30% being largerthan 0.2 mm, preferably the particles are within the range of about 5×0mm, most preferably a 3×0 mm size fraction but a size fraction of 1×0 mmand includes fines is also suitable. The MgO constituent of the deadburned magnesite and the light burned magnesite may be replaced withburned dolomite aggregate all or in part. The smaller crystals of MgOoccur in light burned magnesite particles and comprise at least 80% andnot more than 97% MgO in magnesite particles less than 100 mesh,preferably less than 200 mesh to promote the desired ease of dissolutionin the slag bath occurring throughout the refining of a heat of steel.The dead burned aggregate may consist of dead burned dolomite and theslag conditioning mixture further include light burned dolomite, eachproviding a sources of CaO and MgO components to the chemistry of theslag to reduce the sulfur content of the refined molten steel.

The 20% to 90% by weight of burned aggregates in hopper 100 arecomprised of two constitute parts, first part are in a size fraction ofless than 8 mm with at least 30% of the aggregates being 0.2 mm orgreater and containing 35% to 94% MgO, preferably between 80% and 94%MgO and the aggregates of the second part are in a size fraction of upto 50% light burned magnesite containing more than 85% MgO and having aparticle size less than 100 mesh and more particularly about 80% or moreparticles less than 200 mesh. The two constitute parts are separatelymeasured by weight and then loaded into a hopper 10. Dead burnedmagnesium oxide fines can be used beneficially to lower ignition lossesand replace light burned magnesite as a component in a slag conditionerin the briquette form.

Carbon from the dense agglomerates or briquettes of this invention, orparticles derived from such agglomerates, react in a more efficient wayin the steel making process in an electric furnace including a veryeffective reducing of slag components to increase the yield of metalssuch as iron from iron oxides normally found in steel slag. The slagmaking additive can be carbonaceous, preferably having carbon contentbetween 78% and 99.8%, and/or the additive can be a slag makingcompatible filler. The measured quantities of burned aggregate and slagadditive in hopper 100 and the binder in hopper 102 are loaded into asuitable mixer 104, such as a muller, ribbon, or auger mixer. The mixer104 is operated for at least two minutes until the aggregates and binderare uniformly dispersed and tempered to form a moldable mass. Thetempered mass is then loaded into an agglomerating machine 106, such asa high-pressure briquette press to produce solid 60 mm square briquettesbetween 30 to 40 mm thick. The briquettes of slag conditioner of thepresent invention may be formed in other sizes, such as 30×30×10 mm;40×40×20 mm; 60×40×20 mm; 70×50×40 mm. Other suitable forms of machines106 for forming agglomerates are a mechanical press, a hydraulic press,a friction screw press, a rotary press, an inclined pelletizing disc andan extruder, all per se well known in the art. The briquettes develop anadequate strength for handling after curing and partially drying instorage room 108 maintained at temperature suitable to promote bondingby operation of the binder and evaporation of residual water, when thebinder is aqueous, for example about three days. The density of thebriquettes typically exceeds 1.8 g/cc and attains a crushing strengthmeasured according to ASTM test methods modified for 2 cm cubes, toexceed 2000 pounds per square foot. The resulting agglomerates aresuitable for charging into an electric furnace with iron bearing scrapcharge and fluxes such as burnt lime to alter the chemistry of the slagoccurring during the steel making operation. Additionally or alternativethe agglomerates maybe crushed to a size suitable for injection into thedeveloping layer of slag overlying the steel burden in the furnace.

The present invention retains the benefit of slag conditioningbriquettes or aggregates of the present invention to maintain thedesired MgO content in the slag throughout the steel making operation.The slag appearance exhibiting a creamy texture is a reliable indicatorof a surplus of solid MgO crystals which increase the bulk viscosity ofthe liquid slag. Foaming of the slag by the injection of surprisinglysmaller quantities of carbon is sufficient to produce a reaction withthe oxygen blown into the furnace or by reaction with FeO in the slag torelease CO and CO₂ gases to cause a slag to foam. However, when thecarbon particles are supplied by the briquette, the carbon is altered tothe form of dense particles that penetrate deep into the slag bath sothat the reaction with FeO or oxygen creates gas in a position to betterfoam the slag. The role of the carbon component of the briquettes ofthis invention, when used in conjunction with the correct type and sizeof MgO source materials, is associated with a particle of high densityin the briquette. Even when the briquettes are crushed and injected asfines, the carbon is associated with a dense, but a finer particle size.The carbon associated with higher density particles from the briquettecompositions is consumed very efficiently in the steel making process.The briquettes of this invention provide between 8.5% to 12% MgO to theslag.

Taking into account the observation that both MgO and carbon was beingprovided to the steel making process more efficiently through the use ofthe agglomerate of finer materials in the form of a briquette, it wasdiscovered that the purity of the MgO source, the particle size and thedensity of the grains are important factors controlling the solubilityof the MgO units into the slag. Therefore, the dead burned magnesiumoxide used in the slag conditioners of this invention shall be not morethan 94% MgO purity, and have a grain bulk density or bulk specificgravity of not more than 2.25 g/cc. This observation explains whycrushed used refractory brick especially brick containing fused MgO ofany size does not provide the expected benefits of slag conditioners,even compared to dolime, dead burned magnesium oxide 15×3 mm.

The improved slag conditioner of the present invention using bag houseinkling dropout box particulate contributes to the needed MgO and limeunits in the slag The charge weight of briquette material can bereduced. Therefore, the weight of MgO units added to the furnace isreduced while the same MgO level can be realized in the slag. Thebriquettes also permitted a reduction to the burnt lime added to thefurnace. This in turn lowered the CaO and therefore the CaO to SiO₂ratio. Liquid slag at a lower CaO to SiO₂ ratio is known to favor lowerFeO in the slag. More iron could therefore be converted to steeltherefore increasing the yield of tapped steel. The slag conditioner maybe introduced to electric steelmaking furnace in an amount needed toraise the MgO level in high lime calcium-silicate slag to between 5% to14% , although for low lime calcium silicate slag up to 18% is usefuland thereby impart a creamy slag texture, non leaching for soluble MgO,foam producing to increasing slag volume, and protectively coatrefractory sidewalls of the electric steelmaking furnace. The slagconditioner is introduced in an electric furnace in sufficientquantities to raise the MgO level in a high lime calcium-silicate slagto up to 22% when the CaO to SiO₂ ratio is below 1.5. In the event ofthe failure to develop a creamy slag texture and the slag has theappearance of thin water like texture or consistency and further theslag does not foam well, the quantity of slag conditioner introduced isselected as an amount sufficient to raise the MgO level in a high limecalcium-silicate slag to up to 14% by adding more burned lime toincrease the CaO to SiO₂ ratio to between 1.8 and 2.1. The charging ofthe furnace may includes charging an iron bearing metal at two differentintervals of time into and between 20%-80% of the slag conditioner ischarged during a furst of the two internals of time and between 20%-80%of the slag conditioner is charged during a second of the two intervalsof time. Typically, at least 20% of the calculated slag conditioner ischarged during a time of heating the iron bearing metal in the furnace.In addition, an objective is to combine other compatible additives intothe briquette slag conditioner with or without carbon, to provide finermore reactive materials efficiently to the steel or to the slag.Additives introduced in this manner may include silicon carbide andferrosilicon for reducing other valuable oxides to metal such as chromicoxide to chrome metal in the manufacture of stainless steel.Alternately, burned dolomite can be added to provide finer more reactiveparticles of CaO and MgO. Incorporating the finer materials in abriquette form ensures the materials will reach the slag bath interfaceand be reacted efficiently.

While the present invention has been described in connection with thepreferred embodiments of the various figures, it is to be understoodthat other similar embodiments may be used or modifications andadditions may be made to the described embodiments for performing thesame function of the present invention without deviating there from.Therefore, the present invention should not be limited to any singleembodiment, but rather construed in breadth and scope in accordance withthe recitation of the appended claims.

1. A slag conditioner comprising by weight a mixture and 2% to 25%binder for bonded agglomerates or larger particles of said mixture, saidmixture comprising: 10% to 60% particulates comprised of bag house dustessentially containing oxides of iron, calcium, silicon, magnesium,zinc, lead and cadmium; 20% to 90% burned aggregates comprised ofparticles less than 8 mm of which at least 30% is 0.2 mm or greater andcontaining between 35% and 94% MgO; up to 50% slag-making carbonaceousadditive; and up to 50% light burned magnesite.
 2. The slag conditioneraccording to claim 1 wherein said particulates comprised comprisebetween 10% and 30% of bag house.
 3. The slag conditioner according toclaim 1 wherein said particulates comprised comprise between 10% and 25%of bag house.
 4. The slag conditioner according to claim 1 wherein saidparticulates comprised of bag house dust comprise particles in the rangeof 0.1 and 1000 microns.
 5. The slag conditioner according to claim 1wherein said particulates comprised of bag house dust includeparticulates of drop out boxes essentially including oxides of iron,calcium, silicon, magnesium, zinc, lead and cadmium.
 6. The slagconditioner according to claim 1 wherein said burned aggregate isselected from the group consisting of: dead burned MgO; light burnedMgO; burned dolomite; crushed magnesite carbon brick; fines of a bondedmixture containing MgO and carbon and fines of said mixture.
 7. The slagconditioner according to claim 6 wherein said carbonaceous additive isselected from the group consisting of: coal; coke; petroleum coke;crushed magnesite carbon brick; ladle slag line brick, steel makingfurnace brick; fines of a bonded mixture containing MgO and carbon andfines of said mixture.
 8. The slag conditioner according to claim 1wherein said carbonaceous additive is selected from the group consistingof: coal; coke; petroleum coke; crushed magnesite carbon brick; ladleslag line brick, steel making furnace brick; and fines of said mixture.9. The slag conditioner according to claim 1 wherein said burnedaggregate comprises particle less than 8 mm of dead burned magnesitecontaining between 80% and 94% MgO.
 10. The slag conditioner accordingto claim 1 wherein said burned aggregate comprise particles in a sizerange of 6×0 mm.
 11. The slag conditioner according to claim 1 whereinsaid burned aggregate comprise particles 3×0 mm.
 12. The slagconditioner according to claim 1 wherein said burned aggregate compriseparticles 1×0 mm including fines.
 13. A method of making steel includingthe steps of: recovering particulate emissions containing oxides ofiron, calcium, silicon, magnesium, zinc, lead and cadmium from aparticulate containment system including bag house of a electric steelmaking furnace; producing a slag conditioning by forming agglomeratescomprised of a mixture of aggregates and 15% to 60% of said particulateemissions obtained by said step of recovering and 2% to 30% binder toagglomerate said mixture, said mixture comprising: by weight 15% to 80%dead burned magnesite comprised of particles less than 8 mm of which atleast 30% is 0.2 mm or greater and containing between 35% and 94% MgO;up to 40% light burned magnesite; and 5% to 50% carbon selected from thegroup consisting of: coal; coke; graphite and petroleum coke and ironoxide; introducing an iron bearing charge into said electric steelmaking furnace; melting and refining an iron bearing charge in saidelectric furnace while forming an overlying layer of slag; introducingsaid slag conditioner to said electric steelmaking furnace in an amountneeded to raise the MgO level in said overlying layer of slag to between5% to 14% and thereby impart a creamy slag texture, non leaching forsoluble MgO, foam producing to increase slag volume, and protectivelycoat refractory sidewalls of said electric steelmaking furnace.
 14. Themethod of making steel according to claim 13 wherein said step ofintroducing a slag conditioner includes introducing briquettes, producedby said step of forming, to said electric steel making furnace duringsaid step of introducing an iron bearing charge.
 15. The method ofmaking steel according to claim 13 wherein said step of introducing aslag conditioner includes introducing briquettes produced by said stepof forming to said electric steel making furnace during said step ofmelting and refining an iron bearing charge.
 16. The method of makingsteel according to claim 13 including the step of crushing saidagglomerates to form slag conditioner particulates for said step ofintroducing a slag conditioner, and wherein said step of introducing aslag conditioner includes injecting said slag conditioner particulatesinto said overlying layer of slag.
 17. The method of making steelaccording to claim 13 wherein said particulates comprise between 10% and30% .
 18. The method of making steel according to claim 13 wherein saidparticulates comprise between 10% and 25%.
 19. The method of makingsteel according to claim 13 wherein said particulates comprise particlesin the range of 0.1 and 1000 microns.
 20. The method of making steelaccording to claim 13 wherein said particulates include particulates ofdrop out boxes essentially including oxides of iron, calcium, silicon,magnesium, zinc, lead and cadmium.